Stable non-hazardous solid acid catalysts are widely sought after as replacements for liquid acid catalysts, such as sulfuric, hydrofluoric, and nitric acids. Solid acid catalysts can be used as heterogeneous catalysts in liquid-phase reactions, and are advantageous because they can be easily separated from liquid reaction mixtures. In addition, in view of corrosive and toxic nature of commonly used liquid acids, the environmentally benign non-corrosive solid acids appear particularly attractive. A variety of solid acids have been developed to address these needs, including sulfonated zirconia, ZrO2/WO3 catalyst, zeolites, acidic ion-exchange resins, and silica-supported Nafion® acid resin (e.g., SAC-13, commercially available from Sigma-Aldrich, Milwaukee, Wis.). However, activities and stabilities of these catalysts are often insufficient for commercial use.
A solid acid catalyst could be useful in the process of fatty acid methyl ester manufacturing. Fatty acid methyl esters have acquired considerable commercial significance as starting materials for production of biodiesel fuels, fatty alcohols and other oleochemical products, such as ester sulfonates, and fatty acid alkanolamides. Industrially, fatty acid methyl esters are produced mainly by catalytic transesterification (alcoholysis) of fatty acid triglyceride mixtures, obtained from fats and oils of vegetable and animal origin. However, natural fats and oils almost always contain considerable amount of free fatty acids (FFAs), which readily form soaps upon exposure to alkaline reagents commonly used during transesterification. Formation of soaps significantly complicates isolation of resulting methyl esters, and therefore should be avoided. Reduction of FFA content can be performed, for example, by converting the FFAs into the corresponding alkyl or glycerin esters in a preliminary pre-esterification reaction.
Recently, a number of solid catalysts were suggested for FFA pre-esterification, including acid resins and silica-supported Nafion® acid resin. However, acid resins have low thermal stability, poor mechanical properties, and are also subject to swelling. SAC-13 catalyst has relatively low activity and relatively low thermal stability.